Thermal control of voice coils in loudspeakers

Abstract

In an embodiment of the invention, the voice coil of an electro dynamic transducer is protected against thermal overload by estimating the temperature of a magnet in the electro dynamic transducer. When a power limit based on the temperature of the magnet and on a predetermined voice coil temperature limit is reached by an audio signal, the power applied to the voice coil is reduced.

What is claimed is: 
     
       1. A method for protecting a voice coil in an electro dynamic transducer against a thermal overload, comprising:
 estimating a temperature of a magnet in the electro dynamic transducer; 
 reducing a power applied to the voice coil when a power of an audio signal exceeds a power limit P lim ; 
 wherein the power limit P lim  is approximately determined by the temperature of the magnet and a predetermined voice coil temperature limit; 
 wherein the power limit P lim  is proportional to:
   ( T   lim   −T   m )/ R   tv    
 
 wherein T lim  is the predetermined voice coil temperature limit; 
 wherein T m  is the temperature of the magnet; 
 wherein R tv  is the thermal resistance of a path from the voice coil to the magnet; and 
 wherein when the power limit P lim  is equal to or less than zero, no power is applied to the voice coil; 
 wherein when the power limit P lim  is a positive number, the power applied to the voice coil is dependent on a value of P lim  and the power of the audio signal. 
 
     
     
       2. The method of  claim 1  wherein a value of R tv  varies with the temperature T m  of the magnet and the temperature of the voice coil. 
     
     
       3. An apparatus for controlling a temperature of a voice coil in an electro dynamic transducer comprising:
 an amplifier, the amplifier having an input and an output wherein the voice coil is electrically connected to the output of the amplifier; 
 a DAC having an output and an input wherein the output of the DAC is electrically connected to the input of the amplifier; 
 a dynamic power limiter, the dynamic power limiter having two inputs and an output, the output of the dynamic power limiter electrically connected to the input of the DAC; 
 a first resistor having first and second nodes; wherein the resistance of the first resistor changes resistance with changes in temperature of a magnet contained in the electro dynamic transducer; 
 a resistive network having two inputs and two outputs wherein the two inputs are electrically connected to the first and second nodes of the first resistor; 
 an ADC having inputs and outputs wherein a first analog voltage is presented at the inputs of the ADC; wherein the first analog voltage is a voltage across the two outputs of the resistive network; wherein a first output from the ADC is a digital representation of the first analog voltage; 
 a temperature estimator, the temperature estimator having an input and an output, wherein the digital representation of the first analog voltage is presented at the input of the temperature estimator; 
 a control logic circuit, the control logic circuit having an input and an output wherein the input of the control logic circuit is electrically connected to the output of the temperature estimator and the output of the control logic circuit is electrically connected to a first input of the dynamic power limiter; 
 wherein a digital audio signal is directed to a second input of the dynamic power limiter; 
 wherein power applied to the voice coil is controlled by a power limit P lim  and a power of the digital audio signal; 
 wherein the power limit P lim  is proportional to:
   ( T   lim   −T   m )/ R   tv    
 
 wherein T lim  is the predetermined voice coil temperature limit; 
 wherein T m  is the temperature of the magnet; 
 wherein R tv  is the thermal resistance of a path from the voice coil to the magnet; and 
 wherein when the power limit P lim  is equal to or less than zero, no power is applied to the voice coil. 
 
     
     
       4. The apparatus of  claim 3  wherein the value of R tv  varies with the temperature T m  of the magnet and the temperature of the voice coil.